Democratic leaders are expected to take on the monumental challenge of getting the Senate to act on global warming today by formally unveiling a draft climate change bill proposing a 20% cut in greenhouse gas emissions.

The draft bill, which is to be announced by Senators Barbara Boxer and John Kerry at a press conference this morning, sets out a more ambitious target for cutting greenhouse gas emissions than the 17% cut from 2005 levels by 2020 passed by the House of Representatives in June.

The draft would push for a 20% reduction from 2005 levels by 2020 and an 83% reduction by 2050.

But it is far from clear that the Senate will be able to pivot from its battles over healthcare to climate change and make significant progress before the Copenhagen meeting.

The 800-page draft bill, which is still being worked on, is almost certain to undergo significant changes in the coming weeks with Democrats struggling to build support even from within their own ranks.

"Complex processes are part and parcel of passing major legislation," Tony Kreindler of the Environmental Defence Fund said in an email to reporters on Tuesday. "The most important thing is that the draft be taken for what it is: a starting point that Senators can work with, tailor and pass."

Boxer and Kerry plan a high-profile launch, with fellow Senators, environmental activists, and executives of some of the household name firms that have been pushing for climate change legislation at the press conference.

Republicans, who are expected to largely oppose the bill, are planning their own counter-press conference. On Tuesday, Republican members of Boxer's environment and public works committee wrote a letter warning they would not be rushed into a vote on the bill.

Today's formal start of the legislative process on climate change in the Senate has assumed huge importance in the run-up to meetings in Copenhagen.

World leaders see movement on a US climate change bill as essential to getting an international deal. A series of delays on introducing the bill to the Senate — plus mixed signals from Barack Obama and other members of his administration on the need for urgent action — has deepened fears that the talks at Copenhagen could end in deadlock.

Those fears increased this month when the Senate majority leader, Harry Reid, said the Senate was so taken up with healthcare that it might wait until 2010 before it even got to climate change.

But the administration is coming under pressure from outside the US to make significant steps on emissions reductions. "If we don't come to an agreement in Copenhagen this year American business will suffer the most," said Connie Hedegaard, the Danish minister for climate and energy who as one of the hosts will be heavily involved in the negotiations.

The course of climate change legislation in the Senate is expected to be even more difficult than in the house, where 44 Democrats defied party leaders and the White House to vote against the bill. In August, 10 Democratic Senators demanded that any climate change bill would protect workers in oil and coal states.

Senate Democrats had been expected to further water down their bill to try to secure support from conservative Democrats in the rust belt and coal producing states, but drafts circulating on Capitol Hill this week defied some of those predictions.

Aside from the more robust cuts in emissions, the draft would restore the Environmental Protection Agency's authority to regulate carbon from coal power plants, which had been eliminated under the house bill.

However, the draft includes other measures which could trouble environmentalists. It raises the possibility of trade sanctions against countries that do not cut their emissions. Negotiators have warned such a move could complicated efforts to reach a deal at Copenhagen, especailly with rapidly industrialising countries such as India and China.

The Senate draft also appears to give an opening to an expansion of nuclear power — a bow to Senate Republicans who have been clamouring for 100 more nuclear plants.

Such gestures are part of a strategy aimed at broadening support for the bill. For months Kerry has been inviting fellow Senators to Tuesday morning breakfast meetings with environmentalists and business leaders supporting climate change legislation.

Democratic leaders have also borrowed the strategy deployed by Henry Waxman and Ed Markey, the authors of the house bill, by refusing to specify their plans for the distribution of emission allowances — essentially licences to pollute that energy and manufacturing firms would be compelled to purchase under climate change bill. The omission is intended to avoid a confrontation over the distribution of valuable permits.

Environmental organisations and business leaders have also been pushing hard to cultivate support for a bill, releasing a battery of reports showing the economic and job benefits of the shift to a cleaner energy economy.

The University of California at Berkeley, said in a report published today that the American Clean Energy and Security Act passed in the house in June would create up to 1.9m jobs by 2020.